1. Field of the Invention
The present invention relates to a light-emitting device that uses a luminous organic film. Further, the present invention relates to electric equipment using the light-emitting device as a display portion or a light source. It is to be noted that the luminous organic film, which can be used in the present invention, includes all organic films that emit light (fluorescent light and/or phosphorescent light) via either a singlet excitation or a triplet excitation, or via both excitations.
2. Description of the Related Art
In recent years, development is proceeding in a light-emitting device (hereinafter referred to as an EL light-emitting device) employing a luminous element (hereinafter referred to as an EL element) that uses a luminous organic film (hereinafter referred to as an organic EL film) that provides EL (Electro Luminescence). The EL light-emitting device has an EL element that is composed of an anode, a cathode, and an organic EL film sandwiched therebetween. The emission of light can be attained by applying a voltage between the anode and the cathode.
At this point, a hole from the anode is injected into the EL material, and an electron from the cathode is injected therein. Electric charges (carriers) injected from both the electrodes move in the interior of the organic EL film to thereby re-couple. An excitation state is generated by the re-coupling of the carriers, and a portion thereof is converted into photons. Luminescence can be made visible by extracting these photons to the outside.
Such a conventional light-emitting mechanism of the EL element is shown in FIGS. 2A and 2B. Shown in FIG. 2A is the conventional junction structure of the EL element in which reference symbol 201 denotes a cathode, reference symbol 202 denotes an electron transfer layer, reference symbol 203 denotes an emission layer, reference symbol 204 denotes a hole transfer layer, and reference symbol 205 denotes an anode. Further, shown in FIG. 2B is the carrier injection process thereof. A voltage is applied between the cathode 201 and the anode 205 to thereby inject an electron 206 and a hole 207. The injected electron 206 and hole 207 re-couple, whereby an emission 208 is attained.
Taking into consideration such a light-emitting mechanism, the efficiency of light emitted from the EL element, that is, the emission efficiency (expressed as η (emission)) is expressed as the following equation.η(emission)=η(injection)×η(re-coupling)×η(excitation)×η(quantum)
Here in this equation, η (injection) denotes the efficiency when the carrier is injected from the electrode, η (re-coupling) denotes the re-coupling efficiency of the electron and the hole, η (excitation) denotes the efficiency of generating a singlet exciton due to the re-coupling, and η (quantum) denotes the efficiency of converting the singlet exciton to a photon.
The η (injection) efficiency originates in an electric potential barrier in the interface between the cathode (or the anode) and the EL material, and changes. The lower the electric potential barrier, the higher the η (injection) efficiency is. The η (re-coupling) efficiency changes due to the injection balance of the carrier (balance of the ratio of the injected electron and hole), and is influenced by the carrier transfer characteristic of the emission layer (the organic EL film that will actually emit light). Further, the η (excitation) efficiency is the generating efficiency of the singlet exciton that contributes to the emission of light, and is theoretically set (fixed) at about 0.25. Further, the change of the η (quantum) efficiency depends on whether the emission layer is crystalline or non-crystalline. Generally speaking a higher value can be attained from a crystalline emission layer than from a non-crystalline one.
In addition, until the photons, which are generated in the emission layer, are extracted to the outside, most of them are lost (about 80% are lost) due to diffusion and thermal deactivation. Therefore, light that is actually observed includes the loss of the photons. Thus, in the light-emitting mechanism process of the EL element, the emission efficiency is reduced due to various factors. In order to obtain high emission efficiency, the above-mentioned various efficiencies have to be raised to thereby attain a total high emission efficiency.